A knock control system intended to control a knocking phenomenon, increase engine torque, and improve fuel consumption by sensing a knocking phenomenon in an engine of a vehicle or the like, conveying the presence or absence of the knocking phenomenon to an engine control unit (ECU), and controlling the ignition timing of spark plugs within cylinders of the engine with the ECU is known conventionally. This knock sensor senses vibration characteristic to the knocking phenomenon, and the vibration detector thereof has conventionally used a piezoelectric element composed of ceramic.
Broadly speaking, there are two types of detection methods for knock vibration of this knock sensor. One is a resonance type which causes a piezoelectric element to resonate together with a knocking frequency and detects output due to the resonance thereof as a knock signal, as is described in Japanese Patent Application Laid-open No. 62-96823 Patent Gazette, Japanese Patent Application Laid-open No. 59-164921 Patent Gazette, or Japanese Utility Model Application Laid-open No. 62-128332. The other is a flat type which detects a knock signal in a flat region in which an output signal output by a piezoelectric element is not subject to the influence of resonance, as described in Japanese Utility Model Application Laid-open No. 57-99133. Because the former causes resonance with the knock vibration, output with a good signal-to-noise ratio is obtained, but conversely only a unique knock vibration can be detected, and in the case of an engine with many cylinders it is impossible to detect knock vibration of all cylinders at a single location, and the problem exists that a plurality of knock sensors are required. On the other hand, the latter can detect knock vibration of various frequencies, but the possibility exists that, aside from the influence of the resonant frequency of the element itself, the vibration of other components may exert an influence on the vibration detection region, and there exists the problem wherein the degree of freedom in design of the knock sensor itself is narrow.
As the structure of these sensors, a structure disposing a piezoelectric element composed of a vibration detector within a space formed by a housing made of metal (or a housing composed of a strong material to be substituted thereby) having a projection of screw configuration so as to be installed directly on an engine and of a connector molded of resin which allows connector connection with an external portion is common.
Accordingly, there are two types of piezoelectric element installations: a type firmly fixed to the housing side by means of a screw or the like, and a type fixed in a state fixed to a stem of metal which becomes a fixing pedestal (or a strong fixing pedestal to be substituted thereby) on the connector side. Additionally, for the latter there exist, as similar types thereof, a type fixed to the stem, and not fixed directly to the connector side but fixed by means of caulking, and a type wherein the contact point of the step and housing are connected by means of gluing or welding or the like. That is to say, the latter can be termed a type fixed to the stem and disposed within a spaced formed by the stem and housing.
In a case where the piezoelectric element is fixed to the housing side, resonant frequency is high because the housing itself is made of metal, the housing itself does not resonate due to engine vibration, and influence thereof is not exerted on the piezoelectric element. However, it is necessary to perform the electrical connection from the piezoelectric element to the connector terminal by means of for example lead wires, the connector and housing must be fixed by means of caulking or the like to fix the piezoelectric element to the housing and connect the lead wires, and there exists the problem of a difficult fabrication process.
On the other hand, in a case of fixing to the connector side, because electrical connection from the piezoelectric elements to the connector terminal can be performed at the connector side, it is sufficient to fix a connector whereon a piezoelectric element is fixed to the housing by means of caulking or the like, and the fabrication process becomes simple. However, because the connector is generally made of resin, the resonant frequency is low, and in a case whereby the piezoelectric element is connected to the connector with nothing, resonance of the connector is conveyed without being attenuated by the piezoelectric element, and there exists a problem of influence being exerted on signal detection. To prevent this, mounting on a stem made of metal (or a strong material the Young's modulus of which is not less than metal) so as to impede vibration conveyance of the connector is required.
In this case, with a piezoelectric element of resonant type the detection signal is the resonance output of the piezoelectric element, and so there is no influence if output due to resonance of the connector or the like is to a certain extent smaller than that due to resonance of the piezoelectric element, but with a piezoelectric element of flat type, in a case whereby frequency thereof is a flat region, the signal-to-noise ratio may be caused to decline greatly, and so vibration from the connector must reliably be impeded by making the thickness of the stem thicker. This can be said to be the same also for the type connecting the contact point of the stem and housing by means of gluing or welding or the like. That is to say, with partial welding or the like adequate suppression of resonance of connector vibration cannot be performed.
Accordingly, this problem is subjected to the most influence by the weight of the piezoelectric element itself, and in a case of identical stem thickness, if the weight thereof becomes heavier the resonant frequency of the stem declines, and the influence thereof appears even more strongly. For this reason, it is necessary to cause the weight of the piezoelectric element to be reduced, but if this is done a problem of a drop in sensitivity appeared. Consequently, in order to avoid the influence of a decline in resonant frequency while maintaining sensitivity, the detection frequency region which assumes flat characteristics becomes a maximum of approximately 10 kHz, and the problem exists wherein detection up to a high-frequency region is not possible.
Accordingly, in view of this problem it is an object of the present idea to provide a knock sensor having a vibration detector of flat type capable of detecting a plurality of knock signals and fixed on a fixing pedestal as well as disposing the vibration detector within a space formed by the fixing pedestal and a housing, having a simple fabrication process and moreover capable of detection up to a high-frequency region without causing required sensitivity to decline.
The present inventors firstly made verification regarding the vibration detector. As a result of investigation by the present inventors, it was understood that in a case whereby the detection region of the knock signal is taken to be a maximum of approximately 15 kHz, the resonant frequency of a stem (made of metal) which does not overly influence this detection region becomes a minimum of approximately 40 kHz. When a stem thickness whereby resonant frequency becomes 40 kHz was investigated, it was understood that roughly 2.7 mm was required, as shown in FIG. 20. This is a simulation performed by means of the model indicated in FIG. 22, and this stem 30 takes the diameter thereof to be 19 mm, a region combining the vibration detector and other circuitry thereof is taken to be a load region 31, and the diameter of the surface on which the load region 31 is mounted is taken to be 16.5 mm. Additionally, this stem 30 has a step, and a weld portion M welded to a housing (not illustrated) is formed on the step surface thereof so as to approach actual stem configuration. As can be understood from this drawing as well, if stem thickness D is caused to change without changing the diameter, it is understood that the resonant frequency of the stem rises. This can be understood if it is considered that, wherein the weld portion M is fixed, there exists an image whereby a solid configuration is more difficult to vibrate than a plate configuration.
Additionally, FIG. 21 indicates change in resonant frequency of the stem in cases whereby stem thickness are taken to be 2.8 mm and 3.5 mm and further in a case whereby the load mounted on these stems is caused to change. FIG. 21 is data in a case whereby the load region 31 is caused to change from 0.1 g to 4.6 g. It is understood from this drawing that resonant frequency declines in a case whereby a load is applied to the stem. Consequently, even in a case whereby a load is applied increased thickness is required in order to maintain the resonant frequency at approximately 40 kHz. In a case whereby for example the load is taken to be 4.6 g, in order to make the resonant frequency of the stem to be 40 kHz, when in FIG. 21 the stem thickness is 2.8 mm the resonant frequency thereof is 20 kHz, and consequently it is necessary to double the resonant frequency. Accordingly, if in FIG. 20 the resonant frequency and stem thickness are taken to be in a linear relationship, a stem thickness of 4.4 mm becomes necessary.
In actuality, the weight of a piezoelectric element is approximately 20 g, and in order to cause the resonant frequency of a stem mounted with this piezoelectric element to be 40 kHz, if resonant frequency becomes 10 kHz when hypothetically the stem thickness is 2.8 mm in a case whereby load is taken to be 20 g, the stem thickness must be increased by 3 mm even at a low estimate to approximately 5.8 min.
Consequently, there is not only enlargement as a knock sensor, but in a case whereby a through-hole for the purpose of passing a pin to connect to the connector terminal is made in the stem by punching or cutting, when the strength of the punch pin or cutting drill is considered, it is necessary to make the diameter thereof to be approximately identical to the stem thickness, and it is necessary to form a considerably large through-hole on the stem. In a case of the foregoing stem, if a vibration detector of a piezoelectric element or the like is taken to be mounted on the surface on which load is applied, it is necessary to make a 5.8 mm through-hole on a surface with a diameter of 16.5 mm, and the mounting region of the element is constrained. Consequently, if mounting of signal processing circuitry other than the piezoelectric element is to be attempted, the diameter of the stem must be enlarged. However, enlargement of the diameter of the stem signifies a decline in resonant frequency even if thickness is the same, and stem thickness must be increased further in order to maintain the resonant frequency at the same value. If this occurs, the size of the through-hole must also be enlarged proportionately to the stem thickness as described above, repeating a vicious cycle and generating a failure in which design values are not obtained. Consequently, in a case whereby a piezoelectric element is employed in a vibration detector thereof, the limit for the maximum detection frequency that can be obtained is 10 kHz. In addition, even hypothetically if designed with a large through-hole, the process becomes complex because of the device needed to make the through-hole.
Consequently, in order to enable detection up to high frequencies, it is necessary to use an article lighter than a piezoelectric element. As shown in FIG. 21, even when stem thickness is approximately 3.5 mm, a load allowing the resonant frequency of the stem to be established at 40 kHz is roughly 1 g. If stem thickness is approximately 3.5 mm, formation of a through-hole also becomes possible without major change.
Additionally, even if the fixing pedestal is not a metal stem, ultimately the resonant frequency resonant frequency of the fixing pedestal will undoubtedly decline and exert an influence on vibration detection.
Accordingly, the present inventors gave attention to a semiconductor acceleration sensor formed on a semiconductor substrate used in an airbag and the like as allowing the vibration detector to be made to be 1 g or less. This is structured from a weight (mass), a beam supporting this weight (mass), and a frame to which the beam is fixed by means of etching or the like on for example a semiconductor silicon substrate. The weight (mass) vibrates by means of vibration of an external portion, and vibration is detected by sensing stress generated in the beam by means of vibration thereof; the inventor as well has shipped multiple examples to market. Accordingly, a vibration detector according to this semiconductor is extremely compact with good sensitivity, and thickness thereof can adequately be made to be 1 g or less.